All of mine are registered with Home Again. Thor and Cleo were chipped by the rescue and Khaos by my vet and he also used a Home Again chip. But yes I had to register them to get the number registered. I really don't know all the ins and outs of all the different services but that's what I have.

I have animals with AVID chips, one with a 24PetWatch chip, and some with the Bayer ResQ chip. About the Bayer chip, I like the fact that, once I have paid for the chip, I can register and make changes in the information for life with no charge. By the way, the AVID chips are registered to my vet, not to me. If I want to change them to my name, I'd have to pay. Ilka's ResQ chip is also registered with the AKC's CAR service.

I was stuck with the 24petwatch chip for 2 of mine (I posted about it previously). But to sum it all up, the chip was free to update with no annual fee ever mentioned nor was it in the paperwork. Now 3 years later they said it was only free for the first 60 days and its like $20 a year after that for a maintenance fee. But I updated the chip multiple times after the first 60 days so I'm not very happy with 24petwatch and personally I think they are a scam company. It's one thing to charge. It's completely different to start charging years later and make up some bs story.
My other dog has a resq chip and hers was only a one time fee of $20 in the beginning. I also have them registered through AKC CAR.

I've only ever used HomeAgain and AVID in practice, and both companies keep track of the chips they sell.

I prefer AVID over HomeAgain for the sole reason of HomeAgain trying to get people to pay an annual fee for extra perks which they really don't need - though they market it like you need to do it or your pet won't be found. It just comes off as greedy and misleading to me and I have a hard time morally supporting a company that tries to take advantage of customers like that.

That being said, 3 of my 4 animals are chipped with HomeAgain (since that's what my clinic was using before we switched to AVID), and my last cat is AVID chipped. I registered them all, of course, but do not pay the annual fee for HomeAgain. They are all still registered in the event they got lost and someone called and reported the number.

We've had 2 with Home Again, and 5 with Avid (well, 6, if you count the one I gave my sister for her blind Lab). Fortunately have never had to use them, but I like that the universal code for the Avid chips can be read by any scanner... of course, maybe they are all like that now - I placed those chips 9 years ago, so I'm sure there have been updates in technology since then! With Avid, you can update info; unsure about HomeAgain, since didn't have to do that.